Internal Convection on Ceres: A Possible Explanation for Dome Formation

Numerical 2-D whole-body simulations of the evolution of Ceres' internal dynamics and thermal structure over its history indicate that hydrothermal activity is very strong throughout the first half of Ceres' history, gradually weakening thereafter, but still active even today (Travis et al, 2015, 46th LPSC). Large-scale upwelling plumes of muddy water extend from the porous, permeable rocky core through an ocean layer and impinge on the bottom of the ice shell. These upwellings are very long-lasting. In addition, small scale, shorter-lived plumes frequently develop on the upper regions of the large plumes. The large-scale plumes occur at roughly +/- 25 o latitude. Recently, 3-D simulations of a sector of Ceres shows that the upwellings are indeed plumes and not sheets. In the 3-D model, plume diameters in the model are as small as 15-20 km in diameter, up to several 10s of km or more. Relating internal dynamics to surface features is challenging. Linkage to mounds seen on the surface may be possible. There appear to be two classes of mounds: Large domes (10s of km diameter) and small (<15 km diameter). Morphological evidence such as embayment relations imply that large mounds may be extrusive. The source of the small domes is less clear. They could be extrusive, or they could be pingo-like structures that form when large areas of melt are extruded or produced by impact, although they are larger than terrestrial or martian structures. Mound heights are typically no more than 1 - 5 km. One mechanism for generation of these mounds suggested by our modeling is extrusion of mud through fractures in the icy crust. Over-pressuring of upwelling plumes at the base of the icy crust from freezing of neighboring downwellings could generate fractures in a frozen mud crust. As plumes and icy crust cool, a significant volume expansion occurs due to freezing of water to ice. This pressurization is not uniform in space; the still-liquid upwellings will experience overpressure in our model that may be comparable to or greater than the tensile strength of ice and could create fractures, paths through which plume material could reach the surface. The over-pressure in hydrothermal plumes could be comparable to the overburden pressure at the base of multi-km mounds on the surface of Ceres.